Testing of complex electronic devices requires the reliable communication of signals between a unit under test and a testing assembly. Connections to components to be tested are made using external electrical probes applied to exposed test points (pads, vias, or other electrical contact points) on a unit under test.
Automatic testing of electrical circuits requires simultaneous connection to hundreds or thousands of circuit test points. A conventional test fixture used to provide electrical connection to a unit under test includes the so-called "bed-of-nails" assembly having a platform supporting an array of spring loaded probes. In operation, the unit under test (UUT) is placed on the platform and a force pushing the UUT into contact with the exposed probe ends is exerted, compressing the probes slightly, so as to ensure electrical connection therebetween.
The probes are electrically connected to test equipment in one of two common ways. According to one method, electrical connection is accomplished using fixed wiring. According to this method, dedicated wiring is attached between each probe and the test equipment. The dedicated wiring can take the form of wire wrapping in which a thin gauge wire is physically wrapped around an end of the probe. Alternatively, the dedicated wiring can be connected to the probe by force fitting each wire into a slot in each probe. An example of a probe adapted for this sort of wiring is found in U.S. Pat. No. 5,557,213 to Reuter et al. The labor involved in either method of dedicated wiring makes fixed wiring impractical for testing complex circuitry having thousands of test points. Moreover, this method of providing electrical connection between the test equipment and the probe suffers from a disadvantage in that it is difficult to replace a probe in the event of malfunction.
Alternatively, electrical connection may be achieved using a translator board. According to this method, the bottom probe head (in the case of a double-ended probe) is pressed into contact with a pad (electrical contact point) on a translator board, e.g. a printed circuit board, which is electrically connected to the test equipment, and the top probe head is pressed into contact with a corresponding test point on the UUT. This method for providing electrical interconnection provides many advantages over fixed wiring.
However, this method of providing electrical interconnection poses unique problems. Notably, the force required to maintain good electrical contact between the bottom probe head and the translator board tends, over time, to stress and eventually damage the translator board. See, e.g. FIG. 1.
FIG. 1 shows a conventional wireless assembly 2 used for testing a UUT. The wireless assembly 2 includes a translator board 14, a guide plate 28, and a probe plate 10. The guide plate 28 is supported in a spaced relationship with the translator board 14 by plural spacers 8 (only one spacer shown). In turn, the probe plate 10 is supported in a spaced relationship with the guide plate 28 by plural standoffs 30 (only one standoff shown). Each of the probe plate 10 and the guide plate 28 have corresponding holes configured to receive a double-ended probe socket 20.
The double-ended probe socket 20 is a generally elongated hollow body formed of an electrically conductive material. A spring loaded contact 12 is provided at a bottom end of the double-ended probe socket 20, and an opening configured to receive a probe 22 is provided at a top end thereof. The probe socket 20 is inserted through a corresponding hole defined in the probe plate 10 and guide plate 28. Notably, the guide plate 28 and the probe plate 10 provide lateral support for the probe socket 20. The open end of the probe socket 20 is configured to receive an end of a probe 22.
The probe 22 is a conventional spring loaded electrical contact probe such as disclosed in U.S. Pat. No. 4,814,698 to Johnston et al. The probe 22 has a spring loaded head for making electrical contact with a unit under test 18. In operation, a unit under test 18 is placed on the probe heads such that each head is positioned in contact with a test point 16 on the UUT 18. Next, a force F1 along a longitudinal direction of the probe (shown by a corresponding arrow Fl in FIG. 1) pressing the UUT 18 into contact with the head of the probe 22 ensures firm electrical connection therebetween.
Force F2 is exerted by the bottom head (contact) 12 of the probe socket 20, pushing the contact 12 downward into contact with the translator board 14. To ensure reliable electrical connection, the force F2 must be sufficient to properly compress the spring loaded contact 12 of the double-ended probe socket 20. Force Fl and F2 are independent of each other.
The total force required to compress the spring loaded contact 12 of each of the hundreds or even thousands of probes required to test the complex circuitry of a UUT is considerable. In a conventional assembly 2 this force is transmitted to and tends to cause permanent damage to the translator board.
By manner of example, a typical probe requires approximately four ounces of pressure to compress the springs and provide reliable electrical connection. Thus, a test fixture having 200 probes would require 200.times.4=800 ounces, i.e., fifty pounds of pressure! This force is constantly (statically) exerted upon the translator board and tends, over time, to permanently damage the translator board.
A further problem affecting conventional test assemblies relates to transient forces which are transmitted to the translator board each time a force F1 is applied to the UUT. This problem exists in conventional assemblies despite the use of standoffs. Notably, the transient forces are caused by flexure of the probe plate when subjected to the applied load during testing. Over time the transient force cycles cause wearing of the conductive plating of the pads 17 on the translator board. Ultimately, these transient force cycles result in unreliable electrical connection between the double ended probe socket bottom head 12 and the translator board 14.
Accordingly, a need exists for a test assembly which maintains good electrical contact between a testing assembly and UUT, does not require fixed wiring, and does not exert static forces on the translator board. A need further exists for a test assembly which provides enhanced flexibility without requiring the use of double-ended probes. Still further, a need exists for a test assembly which eliminates or substantially reduces the transient forces exerted on the translator board.